glGetTexImage
returns a texture image into pixels. target specifies whether the desired
texture image is one specified by glTexImage1D (GL_TEXTURE_1D), glTexImage2D
(GL_TEXTURE_2D), or glTexImage3D (GL_TEXTURE_3D). level specifies the level-of-detail
number of the desired image. format and type specify the and type of the
desired image array. See the reference pages glTexImage1D and glDrawPixels
for a description of the acceptable values for the format and type parameters,
respectively.

To understand the operation of glGetTexImage, consider the
selected internal four-component texture image to be an RGBA color buffer
the size of the image. The semantics of glGetTexImage are then identical
to those of glReadPixels, with the exception that no pixel transfer operations
are performed, when called with the same format and type, with x and y
set to 0, width set to the width of the texture image (including border
if one was specified), and height set to 1 for 1D images, or to the height
of the texture image (including border if one was specified) for 2D images.
Because the internal texture image is an RGBA image, pixel formats GL_COLOR_INDEX,
GL_STENCIL_INDEX, and GL_DEPTH_COMPONENT are not accepted, and pixel type
GL_BITMAP is not accepted.

If the selected texture image does not contain
four components, the following mappings are applied. Single-component textures
are treated as RGBA buffers with red set to the single-component value,
green set to 0, blue set to 0, and alpha set to 1. Two-component textures
are treated as RGBA buffers with red set to the value of component zero,
alpha set to the value of component one, and green and blue set to 0. Finally,
three-component textures are treated as RGBA buffers with red set to component
zero, green set to component one, blue set to component two, and alpha
set to 1.

To determine the required size of pixels, use glGetTexLevelParameter
to determine the dimensions of the internal texture image, then scale the
required number of pixels by the storage required for each pixel, based
on format and type. Be sure to take the pixel storage parameters into account,
especially GL_PACK_ALIGNMENT.

If an error is generated, no change is
made to the contents of pixels.

When the GL_ARB_multitexture extension is
supported, glGetTexImage returns the texture image for the active texture
unit.

The types GL_UNSIGNED_BYTE_3_3_2, GL_UNSIGNED_BYTE_2_3_3_REV, GL_UNSIGNED_SHORT_5_6_5,
GL_UNSIGNED_SHORT_5_6_5_REV, GL_UNSIGNED_SHORT_4_4_4_4, GL_UNSIGNED_SHORT_4_4_4_4_REV,
GL_UNSIGNED_SHORT_5_5_5_1, GL_UNSIGNED_SHORT_1_5_5_5_REV, GL_UNSIGNED_INT_8_8_8_8,
GL_UNSIGNED_INT_8_8_8_8_REV, GL_UNSIGNED_INT_10_10_10_2, GL_UNSIGNED_INT_2_10_10_10_REV,
and the formats GL_BGR, and GL_BGRA are available only if the GL version
is 1.2 or greater.

GL_INVALID_ENUM is generated if target, format,
or type is not an accepted value.

GL_INVALID_VALUE is generated if level
is less than 0.

GL_INVALID_VALUE may be generated if level is greater than
$log sub 2 max$, where $max$ is the returned value of GL_MAX_TEXTURE_SIZE.

GL_INVALID_OPERATION is generated if glGetTexImage is executed between
the execution of glBegin and the corresponding execution of glEnd.

GL_INVALID_OPERATION
is returned if type is one of GL_UNSIGNED_BYTE_3_3_2, GL_UNSIGNED_BYTE_2_3_3_REV,
GL_UNSIGNED_SHORT_5_6_5, or GL_UNSIGNED_SHORT_5_6_5_REV and format is not
GL_RGB.

GL_INVALID_OPERATION is returned if type is one of GL_UNSIGNED_SHORT_4_4_4_4,
GL_UNSIGNED_SHORT_4_4_4_4_REV, GL_UNSIGNED_SHORT_5_5_5_1, GL_UNSIGNED_SHORT_1_5_5_5_REV,
GL_UNSIGNED_INT_8_8_8_8, GL_UNSIGNED_INT_8_8_8_8_REV, GL_UNSIGNED_INT_10_10_10_2,
or GL_UNSIGNED_INT_2_10_10_10_REV, and format is neither GL_RGBA or GL_BGRA.

glGetTexLevelParameter with argument GL_TEXTURE_WIDTHglGetTexLevelParameter with argument GL_TEXTURE_HEIGHTglGetTexLevelParameter with argument GL_TEXTURE_BORDERglGetTexLevelParameter with argument GL_TEXTURE_INTERNALFORMATglGet with arguments GL_PACK_ALIGNMENT and others